SUMO1 is transferred from cysteine-173 of UBA2 to cysteine-93 of UBC9 (UBE2I) in a transthiolation reaction (Desterro et al. 1999, Okuma et al. 1999, Tatham et al. 2003, Lois and Lima 2005, Wang et al. 2007, Werner et al. 2009). The UbL domain of E1 recruits E2 into proximity for the transfer of SUMO (Lois and Lima 2005, Wang et al. 2009),
RWDD3 (RSUME) increases noncovalent binding of SUMO-1 to Ubc9 and enhances Ubc9 thioester formation (Carbia-Nagashima et al. 2007).